Quick TB Test Builds Up Arsenal Against Drug-Resistant Bacteria

A medical worker in Carletonville, South Africa, examines a sample at a mobile testing facility for tuberculosis.

Alexander Joe
/ AFP/Getty Images

Originally published on February 2, 2013 4:56 pm

The people on the front lines of tuberculosis control have their hands full, but their biggest challenge for the moment may be containing strains of the disease that are resistant to drugs.

Worldwide the number of TB cases is going down. The bad news is that the number of drug-resistant cases is going up. The World Health Organization estimates that the number of reported TB cases that were multi, extremely- or totally-drug resistant doubled between 2009 and 2011.

Until recently, it was extremely difficult to even diagnose drug-resistant TB, particularly in developing countries where the disease is most prevalent. Tests had to be sent to fancy labs and could take up to three months to process.

But hope arrived a few years ago, in the form of a new screening tool called Xpert. It was designed to identify TB bacteria and the most common form of drug resistance in only two hours. But as a new technology, no one was quite sure how effective it was.

Now, a comprehensive analysis of Xpert's performance, published Thursday in the influential Cochrane Library, validates the test as an accurate tool for detecting resistance to one of the leading TB drugs, rifampicin.

At $17,000 for the machine and $9.98 for each test, Xpert is pricey. But this stamp of approval could encourage more health departments in TB endemic countries to spend the dough on this new method of testing.

In the study, researchers at the University of Washington and McGill University analyzed 18 previously published studies involving almost 8,000 TB patients.

They found that, compared to the conventional methods, Xpert could accurately detect a TB infection 88 percent of the time and rifampicin resistance 94 percent of time.

Health workers in the field are a fan of it, too. Doctors in Cape Town, South Africa working with Doctors Without Borders tell Shots the Xpert is a "game changer" in TB control.

They say that by identifying drug-resistant TB earlier, these highly dangerous strains of the air-borne bacteria can be contained and kept from spreading.

The Xpert machine examines the nucleic acid in a patient's sputum – the mucus coughed up from the upper respiratory tract. The spit is put in a cartridge, which is then stuck into the Xpert machine. Within two hours, a doctor will know if the patient has TB and if that form of TB is resistant to rifampicin.

In the conventional TB test, still widely used today, a medical worker looks for the bacteria under the microscope.

If there are signs of an infection, the sample is then sent out for confirmation by culturing the bacteria in a laboratory. This can take up to six weeks to get results, and testing for drug-resistant strains of the bacteria takes even longer.

TB is the second leading cause of death from an infectious disease after HIV, killing an estimated 1.4 million people around the globe in 2011.